Many touch controllers (also known as "pads", "panels", "tablets", or "digitizers") are known in the prior art. U.S. Pat. No. 4,129,747 (Pepper, Jr., 1978) describes a two-axis pressure-sensitive touch controller that uses a resistive sheet with sinusoidal phase fields established across its surface. In one embodiment, the user directly contacts the resistive sheet, thereby changing the impedance of the resistive sheet at the touch point, however this gives unreliable and unpredictable results because the impedance of the user's body is unpredictable. Another embodiment uses a conductive pickup layer in a sandwich with the resistive layer, which relies only on the area of touch for sensing the pressure dimension, which is also an unreliable technique. Further, both embodiments require complicated electronics to generate two sinusoids in quadrature and accurately detect phase shifts.
U.S. Pat. No. 4,293,734 (Pepper, Jr., 1981) improves on the first embodiment of U.S. Pat. No. 4,129,747, however this device requires rectifiers and analog dividers which reduce the accuracy of the position data.
U.S. Pat. No. 4,644,100 (Brenner et. al., 1987) describes a pressure-sensitive touch panel based on surface acoustic wave propagation over a glass substrate. This technology is expensive to apply; it requires a specialized sensor with etchings and attached transducers, and complicated and expensive electronics. Further, it operates on the principle that the user's finger acoustically dampens the propagating signal, which produces an unpredictable pressure measurement.
Many non-pressure-sensitive touch tablets based on resistive technologies have been described in the prior art, including U.S. Pat. Nos. 4,570,149 (Thornburg et al., 1986); 4,587,378 (Moore, 1986); 4,752,655 (Tajiri et al., 1988); and 4,897,511 (Itaya et al., 1990). Although the present invention has similarities to some of these devices, they do not provide for the sensing of continuous variation in touch pressure.
Of particular interest among non-pressure-sensitive touch tablets are U.S. Pat. Nos. 4,475,008 (Doi et al., 1984) and 4,775,765 (Kimura et al., 1988). Both patents disclose multilayer devices that employ an intermediate pressure-sensitive layer which decreases in resistance locally at the point where applied pressure is increased. However, both devices only use this material in a switching mode and do not provide for continuous sensing of pressure.
U.S. Pat. No. 4,798,919 (Miessler et al., 1989) describes a pressure-sensitive touch tablet based on a single semiconductive resistive sheet, facing a conductive sheet. Applied pressure causes the resistive sheet to reduce its resistance locally at the touch point, while its resistance remains constant everywhere else. The nature of the resistive sheet results in nonlinearity between the true touch position and the reported position. Further, the driving electronics require analog division which further reduce the device's accuracy, and further increase its cost and complexity.
U.S. Pat. No. 4,739,299 (Eventoff et al., 1988) describes a pressure-sensitive touch pad using a pressure-sensitive resistor layer. The touch position is detected using fixed-value resistive sheets, which suffer the same problem of touch position nonlinearity as U.S. Pat. No. 4,798,919.
U.S. Pat. No. 4,810,992 (Eventoff, 1989) discloses another pressure-sensitive touch pad wherein the touch position is linearized within the sensor using a pattern of parallel conductive traces attached to a fixed resistor, in a similar fashion to the earlier non-pressure-sensitive resistive touch pads. However, this solution required the touch sensor to be divided into two overlapping but electrically isolated touch sensors, one sensor for each position dimension. This arrangement reduces the touch sensitivity and reliability of the device because the activating force must press through the upper sensor in order to activate the lower sensor. Further, current flows across the plane of the force variable resistor sheet, rather than perpendicular to the plane, which increases the mechanical contact noise; decreases the standoff resistance; and often exhibits an unnatural force-to-resistance curve. This embodiment also produces two pressure signals, which is more complicated and ambiguous to process than a single pressure signal.
U.S. Pat. Nos. 4,734,034 (Maness et al., 1988) and 4,856,993 (Maness et al., 1989) describe a pressure-sensitive contact sensor which comprises a simple and reliable touch sensor, however the sensor is used in a scanning mode. While this has the advantage of detecting multiple independent touch points, it comes at the cost of requiring an enormous number of terminal contacts from the sensor to the connecting electronics; the scanning hardware is fairly complex and requires a high-speed analog-to-digital converter; the response time is slow due to the time required to complete a scan; very high data rates are produced which incur a computational and memory overhead to process and interpret the data; and the system cost and complexity increase exponentially for linear increases in touch sensor size or resolution. In many applications, only a single touch point need be detected, so that the overhead incurred by scanning is uneconomical.
Copending U.S. patent application Ser. No. 07/497,691 (Asher, filed Mar. 22, 1990), U.S. Pat. No. 5,008,497 describes a touch controller which comprises a current regulator and differential amplifier as an improved electronic circuit for measuring the position and pressure of a touch point on a touch sensor that uses a resistive membrane and force variable resistor. The present invention, however, discloses a touch sensor that represents significant improvements over the prior art of touch sensors. Accordingly, the present invention also discloses two touch controllers that comprise electronic circuits that are further improved and optimized for the new touch sensor.